1. Field of the Invention
The present invention relates to a switching regulated power supply, the output voltage of which is regulated by a pulse width modulation method.
2. Background Art
Switching regulated power supplies are known which have a transformer, a push-pull type switching circuit connected to the primary coil of the transformer and a rectification circuit connected to the secondary coil of the transformer. FIG. 4 is a circuit diagram showing the basic configuration of this type of switching regulated power supply. Both terminals of the primary coil of a transformer T are connected to a positive polarity power source +B through a pair of switching devices SWa and SWb. An intermediate tap of the primary coil of the transformer is connected to a negative polarity power source -B. The secondary coil of the transformer T is connected to a rectification circuit which is not shown. Switching devices SWa and SWb are implemented in a bi-polar transistor or power MOSFET (Metal-Oxide-Semiconductor type Field Effect Transistor) for example. Two gate pulses, which complement each other, are respectively supplied to switching devices SWa and SWb so that the switching devices SWa and SWb alternately become on-state.
In order to control the above-described switching regulated power supply so that its output voltage level remains constant, a pulse width modulation control method is used. In the switching regulated power supplies using such a method, the DC output voltage of the rectification circuit is detected, and the pulse width of the both gate pulses supplied to the switching devices SWa and SWb is controlled by means of a feedback of the detecting result so that the output voltage of the rectification circuit maintains a target level.
In the case where a load device connected to the output terminal of the switching regulated power supply consumes a large amount of electric power, the pulse width of the gate pulses becomes large. As a result, an emergency state can be created in which both switching devices SWa and SWb are in on-state simultaneously, and the primary coil and the switching devices SWa and SWb form a closed-loop circuit, whereby an extremely large current (hereinafter called "short current") passes through the closed-loop and resulting in an extremely large power-loss. In order to prevent the generation of the short current, the switching devices must be controlled so that one of the switching devices becomes on-state after the other has become off-state. Switching regulated power supplies must provides complicated signal processing including a gate pulse generation function capable of preventing the generation of the above-described short current. For that reason, a specificated bi-polar transistor IC (integrated circuit), for example, .mu.PC494 product by Nippon Electric Co., LTD. in Japan, is employed in switching regulated power supplies in order to execute the above-described complicated signal processing.
The specificated bi-polar transistor IC provides an oscillator, and the production of the gate pulses to be supplied to the switching devices is based on the output clock pulse of the oscillator. However, the upper limit of oscillation frequency of oscillators is low, and the oscillation frequency restricts the switching frequency of switching regulated power supplies to a range of approximately 10 kHz up to 100 kHz. In addition, the bi-polar transistor IC consumes a large amount of electric power. Furthermore, the configuration of the circuit of the specificated bi-polar transistor IC is complicated and therefore costly.